feat(m4): persist the download queue across restarts

The queue/scheduler lived only in renderer memory, so saved/scheduled and other
pending downloads were silently lost on quit (audit M4). Persist them:

- main: queue.ts store (proven cached-atomic createJsonStore, queue.json) +
  queue:list / queue:save IPC; QUEUE_MAX cap.
- shared: PersistedQueueItem = the durable subset of DownloadItem (runtime-only
  progress/speed/eta/sizeLabel/finishing dropped).
- renderer: mirror the persistable items on every queue change (a signature over
  the subset means progress ticks don't re-save; main's jsonStore coalesces the
  writes) and rehydrate on launch via an App useEffect. Pure mappers extracted
  to store/queuePersist.ts and unit-tested.

Restore semantics: downloading -> queued (yt-dlp continues the .part), saved +
scheduledFor survives so the promoter fires when due, paused/error return
actionable; completed/canceled are never persisted. A queueHydrated gate stops a
launch-time store change from wiping queue.json before it's read.

Reconciles ROADMAP.md. typecheck + 268 tests + eslint + prettier green.
(OS-level quit/relaunch cycle is worth a manual smoke test.)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main/queue.ts

@@ -0,0 +1,46 @@
+import { createJsonStore } from './jsonStore'
+import { QUEUE_MAX } from './constants'
+import type { PersistedQueueItem, PersistedQueueStatus } from '@shared/ipc'
+
+/**
+ * Persistence for the download queue (M4). The queue lived only in renderer memory,
+ * so `saved` / scheduled and other pending downloads were silently lost on quit.
+ * The renderer now mirrors its durable items here (via `queueSave`) and rehydrates
+ * them on launch (via `queueList`), so a parked/scheduled download returns.
+ *
+ * Reuses the shared cached-atomic `createJsonStore` (R1/R2/R3), so writes are atomic
+ * and coalesced — the renderer can call `queueSave` freely on queue changes.
+ */
+
+const PERSISTED_STATUSES: readonly PersistedQueueStatus[] = [
+  'queued',
+  'downloading',
+  'paused',
+  'saved',
+  'error'
+]
+
+/** Defensive row validator (a hand-edited or partially-written file can't crash a read). */
+function isValidQueueItem(o: unknown): o is PersistedQueueItem {
+  if (typeof o !== 'object' || o === null) return false
+  const it = o as Record<string, unknown>
+  return (
+    typeof it.id === 'string' &&
+    typeof it.url === 'string' &&
+    typeof it.title === 'string' &&
+    (it.kind === 'video' || it.kind === 'audio') &&
+    typeof it.quality === 'string' &&
+    typeof it.status === 'string' &&
+    (PERSISTED_STATUSES as readonly string[]).includes(it.status)
+  )
+}
+
+const store = createJsonStore('queue.json', isValidQueueItem, QUEUE_MAX)
+
+export function listQueue(): PersistedQueueItem[] {
+  return store.read()
+}
+
+export function saveQueue(items: PersistedQueueItem[]): void {
+  store.write(items)
+}